1. Field of the Invention
The present invention relates to an engine control system and more particularly to an idling speed control system for internal combustion engines. More specifically, the present invention pertains to an idling speed control system of a feedback type wherein the actual engine speed is compared with a reference speed to obtain a difference signal which is used to control the engine intake air so that the actual engine speed approaches the reference speed.
2. Description of Prior Art
In recent automobile engines, there is provided an idling speed control system in which the actual engine speed is compared with a reference speed to control the engine intake air so that the actual engine speed approaches the reference speed. In this type of control system, when the engine is suddenly subjected in an idling operation to a load, such as a motor for a car cooler, the engine speed is momentarily decreased and there may be a danger of engine stop. In order to eliminate the problem, there is proposed by a Japanese patent application No. 53-20902 filed on Feb. 27, 1978 and disclosed for public inspection on Sept. 5, 1979 under the disclosure number 54-113725 to add the signal representing the difference between the actual and reference speeds with a compensating signal which corresponds to the additional load. The proposal is considered to improve to some extent responsive characteristics to load changes of an engine under an idling operation, however, there still is a transient period wherein the engine speed is unstable for a certain time period after a change in load. Describing in more detail, even when the intake air is adjusted by the compensating signal, there is a certain time delay before the engine speed is actually changed, so that there will be a certain amount of decrease in the engine speed and there will therefore be produced a difference signal which will be added with the compensating signal. These signals function to adjust the intake air in the same way so that there is a high possibility of over-control. If such over-control is effected when a load is externally added to the idling engine, the engine will overrun and the engine speed will be abruptly increased. When a load is removed from the engine, such over-control will produce an abrupt decrease in the engine speed and may sometimes cause an engine stop.